In today's energy and environmentally conscious world, high power density and long life expectancy are critical requirements for electronic systems. In a capacitor-supported power electronic system, the dc-link filter or module is usually the dominant part in terms of both volume and cost. Aluminum electrolytic capacitors provide the highest capacitance values of any type of capacitor. However, the life expectancy of electrolytic capacitors is reduced dramatically with elevated ambient temperatures. For every 10° C. increase in temperature, capacitor life is reduced by half. Thus, capacitors are usually a reliability bottleneck in electronic systems. Statistics reveal that up to 30% of electronic system failures are caused by malfunctions of capacitors.
In particular, for high-voltage high-power conversion systems, dc-link capacitors are usually periodically replaced and monitored for reliable and safe operation, leading to substantial maintenance costs. To enhance reliability and lifetime, power film capacitors have been used as replacements for some aluminum electrolytic capacitors in some capacitor-supported applications, such as wind power converters and LED lamp drivers. However, the benefits of using power film capacitors are to some degree compromised by their reduced volumetric efficiency. The volumetric efficiency of electrolytic capacitor is typically ten times higher than power film capacitors. Thus, for the same value of capacitance, the physical size of a power film capacitor is much larger than that of an electrolytic capacitor.
To lessen the dependency of the dc-link capacitors, many prior-art methods have been proposed to minimize the dc-link capacitance. However, the methodology is often dependent on the circuit structure. Moreover, many methods suffer from distorted input current, high voltage stress on auxiliary components or are not well justified in terms of the reduction of the capacitance that can be achieved.
FIG. 1 shows a typical structure of a power electronics system 100 for applications such as power supplies, electronic ballasts, motor drives, etc. It consists of two power converters 102, 104 connected by a dc link 106. The first converter 102 is used to convert the input either in the form of ac or dc into dc. The second converter 104 is used to provide the required form of power to a load 106. For example, an electronic ballast for driving fluorescent lamps consists of a front-stage power factor corrector that converts the line-frequency (50 Hz or 60 Hz) input into a high-voltage dc (typically 400V). Its second-stage is an inverter that delivers high-frequency power to fluorescent lamps. The frequency of the output is higher than 20 kHz. The two stages are interconnected by a dc link. The dc-link voltage is stabilized by a high-voltage dc-link capacitor. The capacitor is used to absorb the instantaneous power difference between the input source and output load, filter the harmonics generated in the first converter, and provide sufficient energy during the hold-up time of the entire system.
Table I of the drawings shows a comparison between an aluminum electrolytic capacitor and a power film capacitor for a dc link in a circuit such as that of FIG. 1. Aluminum electrolytic capacitors are widely chosen because of their high volumetric efficiency and low cost. However, they suffer from the following drawbacks:    1) High equivalent series resistance (ESR) and low ripple current capability. This implies considerable power loss in the capacitor. To handle the ripple current stress, the overall capacitance required is usually much higher than that for limiting the specified voltage variations on the dc bus.    2) Bottleneck of the voltage rating. For applications with high dc-link voltage, two or more electrolytic capacitors are connected in series to sustain a high voltage dc link. Thus, additional circuits are needed to balance the voltage distributions among the capacitors.    3) Relatively short lifetime compared to other components in a power conversion system. The lifetime of the capacitors can be improved by reducing the stress factors such as temperature, operating voltage and ripple current. This implies that the dc-link capacitance should be designed with considerable margins, leading to further increase in the volume and cost, especially for capacitors with high voltage ratings.    4) Considerable maintenance work. The reliability of aluminum electrolytic capacitors is of much concern in power conversion systems. For industrial applications, it usually necessitates to monitor the conditions of the capacitors and replace them periodically.
Advances in film capacitor technology in the last two decades are emerging to be applied for dc-link filtering. Power film capacitors outperform aluminum electrolytic capacitor counterparts in terms of ESR, life expectancy, environmental performance, dc-blocking capability, ripple current capability and reliability. Although low-voltage and high value film capacitors are commercially available, the capacitance of the high-voltage film capacitors still cannot compete with electrolytic capacitors. The major obstacles of power film capacitors for being widely applied for capacitor-supported applications are their relatively low volumetric efficiency and high cost.